Either so-called solid rotors, or a solution using a so-called rotor core packet coupled to a squirrel-cage winding, are generally used in electrical machines. From a constructional point of view, the solid rotor consists of a set of grooves formed in the rotor shaft and a conductive coating material placed in association therewith. The purpose of the construction has been to improve the electrical properties of the machine, especially its efficiency.
The rotor core packet/squirrel-cage winding represents a better electrotechnical solution than the construction having a solid rotor. However, when the rotational speeds are to be increased, the rotor core packet/squirrel-cage winding solution becomes problematic because of the low yield point of the electric sheets. The great number of sheets in the rotor core packet also lowers the bending-critical rotational speed of the rotor. The rotor core packet/squirrel-cage solution is a problematic solution when the manufacturing technique and also the use, especially the balancing, are taken into account.
In a rotor core packet/squirrel-cage winding solution, the drawbacks are more pronounced at high rotational and peripheral speeds of the rotor, making it necessary to use thin and strong sheets. The highest permitted peripheral speed of the rotor construction is proportional to the square root of the yield point of the sheet material. Consequently, the yield point should lie as high as possible when peripheral speeds are increased. The iron losses are proportional to the first and second power of the rotating speed and the second power of the thickness of the packet sheets.
At high rotational speeds, the increased losses are compensated by making the packet sheets thinner. It must be also considered that the packet sheets are not always of equal thickness due to the tolerances in manufacturing technique. Furthermore, they must be provided with a thin insulating layer on their surfaces for decreasing eddy current losses. The rotor core packet made of thin sheets and the squirrel-cage winding must almost always be compressed in an axial direction for achieving the construction entity. This tends to cause stress states or bending in the rotor construction and may result in unbalanced state of the rotor during the use.
It must also be considered that the yield points of the sheets of the rotor core packet are typically lower than the yield points of the material used in the rotor shaft, and the mechanical stresses of the cylindric packet provided with a central hole become relatively high. As a consequence, the ratio of the length to the diameter of the rotor construction becomes disadvantageous, which tends to lower the bending-critical rotation speed and the highest possible available peripheral speed of the rotor. The power factor of an asynchronous electric machine with a rotor core packet/squirrel-cage winding design is good.
On the other hand, electrical machines equipped with solid rotors attain higher peripheral speeds than the above-described rotor core packet/squirrel-cage winding constructions. This usually results in a higher power/weight ratio. In an electrical machine with a solid-rotor design, the power factor is usually poor under load conditions. The magnetic flux in the rotor construction in this case starts to flow increasingly on the outer surface of the rotor construction, and the eddy currents resulting therefrom cause the magnetic flux to accumulate to an ever increasing extent on the surface of the rotor construction. The result is a strong saturation of the surface layer of the rotor and a strong increase in the magnetomotoric force of the magnetic flux. As stated above, the result is a considerable decrease in the power factor. As a general conclusion, the power factor of an electrical machine with a solid-rotor design is poor, which increases to some extent the losses especially in the stator winding and increases considerably the apparent power required by the power feed device.